Emergency power system

ABSTRACT

An emergency power system is connected between an AC voltage source and a load. The emergency power system ensures a stable voltage on the output, even if the AC voltage received from the AC voltage source is irregular or is interrupted briefly. The emergency power system includes an AC/DC converter coupled between the load terminals and an AC regulator positioned between the AC voltage source and the load. The AC/DC converter is coupled to a chargeable battery and serves as a charging rectifier for the battery in normal operation and as an inverter in emergency operation, the battery voltage being converted to AC voltage on the output of the emergency power system. The AC regulator is built as a controllable current generator supplying on its output an AC current which is in phase with the AC voltage received from the AC voltage source. Irrespective of the energy direction through it, the AC/DC converter maintains the predetermined AC voltage (V OUT ) across the output terminals of the emergency power system. The system includes a comparator which senses and compares the battery state to a predetermined reference and supplies a signal in response to this. The intensity of the current supplied by the AC regulator is controlled in response to the signal generated by the comparator so that in terms of energy the system is in power balance.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention concerns an emergency power system of the type defined inthe introductory portion of claim 1.

2. Description of the Prior Art

Emergency power systems of this type are connected between an electricdevice and an AC voltage supply. Many electric devices are now in usewhere mains outage will be destructive to the function of the device.The devices are in particular such as have a built-in execution ofprograms, such as computers, since these are sensitive to mains outagethat may course erasure of data or faulty execution of the programs. Theemergency power systems ensure a stable voltage supply during mainsoutage for a duration of up to several hours.

It is known e.g. from U.S. Pat. No. 4,366,390 to use a battery which ischarged in normal operation and is discharged in emergency operation,and which provides continuous voltage supply to the connected electricdevice or devices until the mains voltage has been re-established. Thebattery is connected to the mains connection through a power converter,which serves as a charging rectifier (charger) in normal operation andserves as an inverter (AC/DC voltage converter) in emergency operation.The power converter moreover stabilizes the voltage on the output of theemergency power system, thereby eliminating fluctuations on the mains.The emergency power system according to this principle is connected witha large choke coil on the input side, which absorbs the differencesbetween the fluctuating input and the stabilizing output without losses.Although voltage stabilization takes place without losses, the chokecoil gives rise to a power factor which varies with the load and themains voltage because the choke coil is reactive.

The structure disclosed in said U.S. patent may be called a parallelpower converter, since a passive series element in the form of the chokecoil cooperates with an active parallel element in the form of theregulating inverter.

SUMMARY OF THE INVENTION

FIGS. 1 and 2 show two well-known structures of UPS systems which bothsupply a consumer or a load Z with a stabilized voltage. Both systemsare connected to an AC voltage generator 10, which may e.g. by a mainsconnection.

The system shown in FIG. 1 operates as a series power converter, saidsystem comprising a charging rectifier 11 which will normally bythyristor-controlled to generate strong harmonics which will be returnedto the mains, unless suitable filtration takes place at the mainsconnection. The charging rectifier 11 converts an AC voltage, suppliedto the input, to an essentially constant output voltage, therebyensuring that a battery 12 is fully charged. This DV voltage isconducted to an inverter 13 which converts the DC voltage to an ACvoltage which is conducted to the load Z. The structure involves twoindividually regulated converters which operate in series. Accordingly,each of the converters is to convert the full output power, resulting ina low efficiency.

FIG. 2 shows another known UPS system which is described in detail inthe U.S. Pat. No. 4,366,390, and which connects an AC voltage source 10a load Z. The emergency power system has a power converter 14 whichoperates as a charging rectifier in normal operation, thereby chargingthe battery 12, and which operates as an inverter in emergencyoperation, thereby converting the battery voltage to an AC voltage whichis supplied to the consumer Z. The emergency power system has a chokecoil 15 which absorbs the differences between the stabilized outputvoltage, supplied to the load Z, and the fluctuating mains voltagereceived from the voltage source 10. The choke absorbs this differencewithout losses, but gives rise to a connection power factor which is afunction of the angle between the emitted voltage and current. Thispower factor should be as close to it as possible owing to the voltagesource, which requires that current and voltage are in phase. However,with a large reactive component such as a choke coil 15, this is notpossible. In addition, the emergency power system comprises a switch 16which is closed in normal operation, and which is opened in case ofmains outage, so that the battery 12 is not caused to operate the mains.

The object of the invention is to provide an improved version of theemergency power system known from said U.S. Pat. No. 4,366,390, whereinthe total efficiency of the system is improved, and wherein thedrawbacks of a varying loss/connection factor are eliminated.

This object is achieved according to the invention by an emergency powersystem which is characterized by the features defined in thecharacterizing portion of claim 1. The choke coil of the prior art ishere replaced by an AC regulator which cooperates with and is controlledby the battery voltage. The current through the AC regulator iscontrolled in response to the need of real power on the output.

Traditional UPS (uninterruptible power supply) systems having a chargingrectifier coupled in series with an inverter, with a battery interposedbetween these, typically have a total efficiency of 85% for a five kVAUPS system. A corresponding UPS system according to U.S. Pat. No.4,366,390 typically has a total efficiency of 91%, but a 5 kVA UPSsystem according to the invention may have a total efficiency of about96%.

The AC regulator is series-connected to the AC voltage source/mains andis controlled so as to constitute an approximately ideal currentgenerator, which means that the AC voltage across the output follows thevoltage across the load, while the emitted current is sinusoidal and hasa curve shape which is identical with the curve shape of the mainsvoltage. This is obtained in that the AC regulator is controlled so thatit can only receive current in phase with the mains, whereby the currentfrom the regulator is in phase with the mains voltage. The AC regulatorhereby just draws real power from the mains, and since it does notcontain essential passive, reactive components, it likewise justsupplies real power to the inverter and the load.

The function of the inverter in normal operation is to stabilize theoutput voltage, i.e. the voltage across the load, and to supply reactiveor harmonic power if so required by the load. Computer equipment oftenrequires supply of sinusoidal currents which have superimposed harmoniccomponents or are phase-shifted from the voltage.

The inverter is therefore coupled so as to constitute an approximatelyideal voltage generator, so that it can maintain the desired sinevoltage across the load and also supply a current contribution on whichthe sinusoidal current provided by the AC regulator is superimposed. Thebattery, together with the inverter, serves as a kind of buffer betweenthe AC regulator and the load, the inverter supplying the differences inpower between the real power supplied by the AC regulator and the powerapplied to the load. The inverter obtains the necessary power from theconnected, chargeable battery. The need for supply of real power fromthe mains to supply the load, to cover losses in the inverter and tocharge the battery, if necessary, is recognized by monitoring the stateof the battery. An insufficient power supply from the mains can bedetected by measuring the battery voltage, since this will drain thebattery of energy, causing a drop in voltage.

It may be said that in the invention, as defined in claim 1, the systemwill be in balance in terms of power, since the mains exclusivelysupplies real power, and the remaining part of the power expended by theload is supplied by the inverter. The remaining part of the powerconsumption is supplied as reactive energy or higher harmonic and isobtained from the battery, which is kept constantly charged byincreasing or reducing the real power supplied from the mains.

It may be said that the function of the emergency power system as aregular standby source has been expanded to also having an importantfunction in operation, viz. to ensure that the load in the form of theelectric device on the output of the emergency power system only drawsreal power from the mains and thereby has a connection factor which isclose to or is equal to one.

The subclaims describe expedient details in an emergency power systemaccording to the invention, and claim 2 thus describes an expedientembodiment in which the AC regulator is divided into two parallelcurrent paths, which are respectively conductive in the positive andnegative, half-periods of the mains voltage. When the current in thecurrent path is regulated, as described in claim 3, the use of reactivecomponents can practically be avoided, and the reactive components usednevertheless are of a size such that they practically do not affect theconnection factor of the emergency power system. Claim 4 states how thepulse generator generates a pulse signal, and the parameters necessaryto obtain this signal. For control purposes the current through the ACregulator may expediently be sensed, as stated in claim 5.

Claim 6 states how the output voltage of the system is kept at a stablelevel, while claims 7 and 8 state the components which the current pathof the AC regulator expediently comprises.

The invention will be explained more fully below in connection with apreferred embodiment and with reference to the drawing, in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 and 2 are diagrams showing the principles of known emergencypower systems.

FIG. 3 is a corresponding diagram showing the principle of a preferredembodiment of an emergency power system according to the invention.

FIG. 4 shows a preferred embodiment of the control part for an emergencypower system according to the invention.

FIG. 5 shows a preferred embodiment of a power converter part for anemergency power system.

FIGS. 6-8 show amplifier configurations used in the power converter partshown in FIG. 5.

FIGS. 9-10 show an equivalence diagram for the emergency power systemaccording to the invention and the currents running in the system,respectively.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 3 shows the fundamental principle of the structure of an emergencypower system according to the invention, and it will seen that thesystem comprises a battery 12 which is charged with an AC voltage, whichis rectified by a current converter 14 serving as a charging rectifierin normal operation. In emergency operation the power converter 14serves as an inverter, the battery voltage from the battery 12 beingconverted to AC voltage which is supplied to the load Z. In replacementof the choke coil 15 in the configuration shown in FIG. 2 the emergencypower system of the invention has an AC regulator 20, which regulatesthe current from the voltage source 10 in response to the need for realpower to the output of the emergency power system. This will bedescribed in detail in connection with the subsequent figures.

FIG. 6 shows a transistor circuit which will be well-known to a personskilled in the art, and which is incorporated as part of the ACregulator in a preferred embodiment of an emergency power systemaccording to the invention. A positive input voltage V₆,IN, which issupplied to the collector input of a transistor T₆, is applied to thecircuit. The base input of the transistor T₆ is controlled by a periodicsignal having a variable duty cycle D, i.e. the part of the period ofthe signal in which a signal level different from zero occurs, can bevaried. A diode D₆, which is conductive in a direction from zero to theemitter of the transistor, is interposed between the emitter input ofthe transistor and zero. An inductance L₆ is connected between theemitter of the transistor and one output terminal of the transistorcircuit. A signal having the input signal as the envelope and having afrequency corresponding to the frequency of the periodic signal appliedto the base of the transistor, will appear on the emitter of thetransistor. Further, the output terminals of the transistor circuit haveinterposed between them a large smoothing capacitor C₆, across whichthere will be a voltage V₆,OUT which has a size determined by V₆,INmultiplied by the duty cycle D. Since the duty cycle will be between 0and 1, such a transistor circuit provides a pulse-controlled amplifierhaving a gain between 0 and 1. The output voltage V₆,OUT may also beexpressed as follows:

    V.sub.6,OUT =V.sub.6,IN D.

FIG. 7 similarly shows another transistor coupling in which the inputhas applied to it a positive input voltage V₇,IN, which is supplied tothe collector on a transistor T₇ through an inductance L₇. Thetransistor T₇ is controlled by a periodic signal having a variable dutycycle D applied to the base electrode, and its emitter electrode isconnected to zero. A diode D₇ is connected between the collector of thetransistor T₇ and one output terminal of the transistor circuit and isconductive in a direction toward this terminal. A large smoothingcapacitor C₇ is arranged across the output, so that the output voltageV₇,OUT can be expressed by the input voltage V₇,IN divided by 1 less theduty cycle D. Since the duty cycle will be between 0 and 1, it will bepossible to obtain a gain greater than 1 for the amplifier, so that theoutput voltage may expressed as follows:

    V.sub.7,OUT =V.sub.7,IN / (1-D).

In FIG. 8 the transistor couplings shown in FIGS. 6 and 7 are coupled toform an integral circuit. The input voltage V₈,IN corresponds to theinput voltage V₆,IN. If the periodic signals controlling the transistorsT₆ and T₇ are identical and in phase, it is not necessary to smooth thesignals till on the output of the circuit, since the transistors T₆ andT₇ will be conductive simultaneously. Accordingly, the capacitor C₆ maybe omitted, and the inductances L₆ and L₇ may be wound together to anintegral inductance L₈. The voltage V₆,OUT corresponds to the voltageV₇,IN, so that the circuit shown in FIG. 8 will be able to supply anoutput voltage V₈,OUT, which can be expressed by the input voltage V₈,INand the duty cycle D. Hence,

    V.sub.8,OUT =V.sub.8,IN D/(1-D).

However, it is frequently preferred to control the two transistorcouplings individually, so that one stage merely transfers the inputvoltage to the output in that the control signal is either high (FIG. 6)or low (FIG. 7), and that the second stage is pulse-controlled. If theinput voltage V₈,IN is to be amplified, the control voltage to thetransistor T₆ is made high, and the transistor T₇ is pulse-modulationcontrolled to provide the desired voltage. Conversely, if the inputvoltage V₈,IN is to be attenuated, the control voltage to the transistorT₇ is made low, and the transistor T₆ is pulse-modulation controlled toattenuate the applied voltage. The capacitor between the two transistorstages can still be omitted, because one transistor merely transfers theinput voltage to its output. When working with different control signalson the two transistors, the output voltage V₈,OUT may be expressed bythe duty cycles D_(T6) and D_(T7) of the control signals. Hence:

    V.sub.8,OUT =V.sub.8,IN D.sub.T6 /(1-D.sub.T7),

where the duty cycle D_(T6) is 1 or D_(T7) is 0 in normal operation,because one of the control signals concerned is either high or low.

FIG. 4 illustrates the control principle for an AC voltage regulatorwhich is shown in detail in connection with FIG. 5. It should be notedthat the control of the power converter part 14 (FIG. 3) is only shownin outline, because it is disclosed in its essentials in the Applicant'sown U.S. Pat. No. 4,366,390.

FIG. 4 shows a synchronization unit 25 which is connected to the ACvoltage supply and controls two sine generators 26, 31, which are keptsynchronous and in phase with the AC voltage supply, and whoseamplitudes can be controlled by applied DC voltages. The voltage fromthe sine generator 26 is applied to the positive input on an erroramplifier 24, whose negative input is connected to a power transformer21 arranged in series with the input terminal of the emergency powersystem. The power transformer 21 is loaded by a resistor 22 to produce avoltage on the negative input of the error amplifier 24, said voltagebeing proportional to the current in the input terminal of the emergencypower system. The signal from the error amplifier 24 is applied to amodulator 23 which generates the modulation signals to the transistors62, 65, 72, 75 shown in FIG. 5. These transistors are modulated so as tomaintain, at any time, the smallest possible or no deviation between thetwo signals applied to the input of the error amplifier 24. If theinstantaneous value of the measured input current has a lower amplitudethan the desired value from the sine generator 26, the duty cycle of thecontrol signals is changed in a direction which increases the gain inthe AC power regulator 20, which increases the input current to thesystem. If, correspondingly, the measured input current has a higheramplitude than the desired value from the sine generator 26, the dutycycle of the control signals is changed in a direction which reduces thegain in the AC power regulator 20 resulting in a reduced input current.This regulation ensures that input currents are always proportional tothe signal from the sine generator concerning amplitude as well curveshape, so that the input current to the system will always be sinusoidaland in phase with the input voltage. This ensures that the emergencypower system can keep a power factor of 1 no matter what happens on theload side, since the power converter 14 compensates for phase shifts, ifany, between current and voltage on the output of the emergency powersystem if the load is reactive.

To control the amplitude on the sine generator 26 and thereby the powerdrawn by the emergency power system from the voltage supply (the mains),the charged state of the battery is monitored. This monitoring isperformed by an error amplifier 28, which receives a measurement valueproportional to the battery voltage on its negative input and receives aconstant reference value V_(REF), by means of which the desired chargedvoltage on the battery 12 is determined, on its positive input. Theerror emplifier 28 controls the amplitude on the sine generator 26 suchthat the lowest possible or no deviations between the two signalsapplied to the input of the error amplifier 28 are obtained. If thebattery voltage is lower than the desired reference value, the voltageto the sine generator 26 is increased, which means that the current andthereby the power received from the mains are increased.

The voltage V_(OUT) on the output of the emergency power system andthereby the power consumed by the load are kept constant, so thatincreased power taken from the mains through the AC regulator 20 canonly be transferred to the battery 12 through the current converter 14,which charges the battery 12, resulting in increased battery voltage.Thus, the entire emergency power system will constantly be in balance interms of power, so that a constant charging voltage is maintained in thebattery 12, no matter whether the input voltage or the load is changed.

The remaining circuits shown in FIG. 4 serve to keep the AC voltage fromthe power converter 14 constant irrespective of changes in batteryvoltage and load. This must be considered prior art, which is employede.g. in the emergency power systems shown in FIGS. 1 and 2. The controlwill therefore just be described briefly below. The signal from the sinegenerator 31 is compared with the actual output voltage V_(OUT) of thesystem in an error amplifier 30. The signal from the error amplifier 30controls the transistors of the power converter 14 through a modulator34 to provide the smallest possible or no difference between the twosignals on the input of the error amplifier 30. To obtain increasedvoltage stability the average voltage of the system output voltageV_(OUT) is measured with an average voltage measuring circuit 32, andthe amplitude of the sine generator 31 is regulated up or down by meansof an error amplifier 33, thereby constantly keeping the average valueof the system output voltage V_(OUT) at a value determined by areference V_(REF) applied to the positive input of the error amplifier33. The modulation signals from the AC regulator 20 as well as the AC/DCconverter 14 have a frequency which is typically between 100-1000 timesgreater than the frequency of the AC voltage applied to the input of theemergency power system.

FIG. 5 schematically shows the design of the emergency power system ofthe invention, the control units being omitted here for clarity, so thatonly the power transferring parts of the system are shown. The ACvoltage regulator 20 is here shown in detail, and it will be seen thatit is built as a Boost/Buck converter. The AC voltage regulator 20 hastwo current branches which are conductive in the positive and in thenegative half-period, respectively, of the AC voltage applied across theinput.

The first one of the current branches accommodates a diode 60 which isconductive in the positive half-period of the applied AC voltage. Afterthe diode 60 a voltage-controlled transistor 62 of the IGBT type(insulated gate bipolar transistor) is connected in parallel with a feedback return path diode 68. The transistor 62 is controlled by a periodicsignal applied to its gate electrode, said signal being generated by themodulator 23 shown in FIG. 4. The duty cycle of this signal can beregulated, which is described in connection with FIG. 4. The emitteroutput of the transistor 62 is connected to the negative electrode ofthe battery 12 via a diode 63. An inductance 64 is likewise connected tothe node between the transistor 62 and the diode 63. It will be seenthat the transistor 62, the diode 63 and the inductance 64 form atransistor configuration which substantially corresponds to the onedescribed in connection with FIG. 6.

In addition, the inductance 64 is connected to another node, betweenwhich and ground a diode 67 is arranged in series with a transistor 65corresponding to the transistor 62 and controlled in the same manner.The diode 67 and the transistor 65 are arranged so that a current canflow from the node with the inductance 64 and the diode 67 towardground. The node between the diode 67 and the inductance 64 is moreoverconnected to one output terminal of the emergency power system through adiode 66, which is polarized so that a current can flow toward theoutput. The diode 67 protects the transistor 65 against oppositelydirected currents and provides, in its conductive state, a voltage dropbetween the node between the transistor 65 and the node between thediode 67 and the inductance 64.

The inductance 64, in combination with the transistor 65, the diode 66and a smoothing capacitor 80 disposed between the output terminals ofthe emergency power system, constitutes a transistor couplingcorresponding to the one shown in FIG. 7. These components, incombination with the transistor 62 and the diode 63, form a transistorconfiguration corresponding to the one shown in FIG. 8. Correspondingly,the other branch of the AC regulator 20 is built in a completelyequivalent manner to the first branch, all unidirectional componentsbeing inverted so that this branch is conductive in the negativehalf-period of the applied AC voltage. Thus, this branch comprises adiode 70 connected to a transistor 72, which corresponds to thetransistor 62, and which is connected in parallel to a feedback diode78. The other node between the diode 78 and the transistor 72 isconnected to the positive electrode of the battery 12 through a diode 73and to an inductance 74, whose other terminal is connected to the outputterminal of the emergency power system through a diode 76 and to zerothrough a transistor 75 and a diode 77. The transistor 75 generallycorresponds to the transistors 65, 62 and 72, and all these arecontrolled by individual control signals--either square signals orhigh/low voltages (on/off).

If a positive half period of the input signal is considered, the currentpaths 60-68 will be conductive. If the input voltage is higher than theoutput voltage, the voltage has to be attenuated, and, cf. thediscussion in connection with FIG. 8, the transistor 65 will receive acontrol signal which is low or off, so that it will be in reverse mode.The transistor 62 receives the pulse width modulation signal and reducesthe voltage in response to this.

When comparing FIG. 5 with FIG. 8, it will be seen that the diodes 63and 73 in FIG. 5 are connected to the negative terminal and the positiveterminal, respectively, on the battery 12, instead of to zero, as wasthe case in connection with FIG. 8. If the diodes 63 and 73 had beenconnected to ground, this would establish an undesirable current pathwhich would short-circuit the output voltage of the system to zerothrough the diodes 63 and 66 and the inductance 64 in the negativehalf-period. Correspondingly, in positive half-periods the system wouldbe short circuited to zero through the diodes 73 and 76 and theinductance 74. To prevent this, the fact that the two transistors 91 and94 in the current converter 14 are always controlled in simple phaseopposition to the output frequency of the power system is turned toaccount, so that the transistor 94 is always conductive and thetransistor 91 is always interrupted in the positive half-period of thevoltage, and conversely in the negative half-period. This ensures thatthe diode 63 has a power current to ground through the transistor 94during the positive half-period in which the diode is to be used. In thenegative half-period the current path through the diode 63 cannot becondutive, since the potential on the negative terminal of the batteryis lower (more negative) than the instantaneous values of the voltagewhich appears on the output of the system. It similarly applies to thediode 73 that it has a current path to ground through the transistor 91during the negative half-period, while it is kept in reverse mode bymeans of the voltage of the battery during the positive half-period.

The transistors 92 and 93 are controlled by pulse width modulated squarevoltages having a high frequency, so that the voltage in the nodebetween the two transistors 92 and 93 forms the desired sinusoidalvoltage on the output of the system, the capacitor 80 together with aninductance 95 forming a lowpass filter. The diodes 80, 84 ensure that nomatter which of the four transistors 91-94 are conductive orinterrupted, there is always a current path from the inductance 95through the battery 12 to the capacitor 80. A current running in theinductance 95 can thus not subject the transistors 91-94 to destructiveerror polarizations. The function of the diodes 81-84 and thetransistors 91-94 may be regarded as general prior art known from thecurrent converters which are shown in connection with FIGS. 1 and 2. Thecontrol of the transistors 91-94 is the same no matter whether thesystem is in normal operation or in emergency operation where the entireAC regulator 20 is dead.

The control principle of the power converter is generally described inthe Applicant's U.S. Pat. No. 4,366,390.

FIG. 5 has been described in outline, because it will e.g. be clear to askilled person that the shown diodes will be provided with a paralleldecoupling capacitor. It will likewise be evident to a skilled personhow the controlled signals are coupled to the base inputs on thetransistors.

It appears from FIG. 9 how the inverter 14 is coupled to the battery 12,across which there is a battery voltage V_(B). The inverter 14 receivesbattery current I_(B) from the battery 12 and supplies a currentcontribution I_(VR) to the load Z. The inverter 14 maintains apredetermined AC voltage V_(L) across the load Z. The current regulatoris indicated as a controllable current generator 20 in FIG. 9, which, inseries with the AC voltage source 10, is connected in parallel to theinverter 14. The variable current generator supplies a current I_(R)which is sinusoidal, as shown in FIG. 10. The voltage V_(L) across theload Z is likewise shown to be sinusoidal in FIG. 10. FIG. 10 shows thatthe load current I_(L) is substantially sinusoidal, but has superimposedhigher harmonic oscillations. The power for the higher harmonicoscillations is provided by the inverter 14, and this can thus supply acurrent I_(VR) corresponding to the one shown in FIG. 10. It will beseen that the current I_(L) across the load Z is esstentially in phasewith the load voltage V_(L), so that in this case the load Z isessentially resistive. If, however, the load Z was partly reactive, theload current I_(L) would be phase shifted with respect to the loadvoltage V_(L). This would not change the curve shape of the currentgenerator current I_(R), but would cause the inverter current I_(R) toassume a sine shape which would be phase shifted with respect to thecurrent I_(R) so that the sum of the currents I_(VR) and I_(R) wouldprovide the current I_(L) necessary for the load.

We claim:
 1. An emergency power system for connection between a pair ofinput terminals for an AC voltage source and a pair of output terminalsfor a load, said emergency power system ensuring a stable voltage acrossthe pair of output terminals even if the AC voltage received from the ACvoltage source is irregular or is briefly interrupted, said emergencypower system comprising:an AC regulator coupled between said inputterminals and said output terminals, said AC regulator including acontrollable current generator for supplying on its output an AC currentwhich is in-phase with the voltage received from the AC voltage source;a chargeable battery; an AC/DC converter coupled to the output of thecontrollable current generator, to the pair of output terminals, and tothe chargeable battery, for serving as a charging rectifier for thebattery in normal operation and as an inverter for converting thebattery voltage to an AC voltage when said AC source is irregular orinterrupted, a control circuit coupled to said pair of output terminalsand to said AC/DC converter, for controlling said AC/DC converter tomaintain a predetermined AC voltage across said pair of outputterminals; and a comparator coupled to said battery and to said ACregulator, for sensing the battery voltage and comparing it with areference voltage, and for providing a control signal for controllingthe intensity of the current supplied by said controllable currentgenerator.
 2. An emergency power system according to claim 1,characterized in that the AC regulator comprises two parallel currentpaths, which are in conductive and in reverse mode, respectively, duringthe positive and negative half-periods of the AC voltage applied to theinput of the system.
 3. An emergency power system according to claim 1,characterized in that the current in the current paths arepulse-controlled by a periodic square signal, and that the systemfurther comprises a pulse generator which generates a square signalwhose duty cycle is regulated in response to the signal generated by thecomparator.
 4. An emergency power system according to claim 3,characterized in that the pulse generator comprises a sine generatorwhich is synchronized with the AC voltage source by a synchronizationunit and is controlled by the signal generated by the comparator, andthat an error amplifier followed by a modulator generates the periodicsquare signal in response to a comparison between the signal generatedby the sine generator and a reference AC voltage signal.
 5. An emergencypower system according to claim 4, characterized in that the referenceAC voltage signal used in the error amplifier is a measure of thecurrent received from the AC voltage source, said measure being providedthrough a current transformer having a load resistance coupled to theinput terminal of the system.
 6. An emergency power system according toclaim 3, characterized by detection means for detecting deviations inthe output voltage of the system with respect to a reference voltage(V_(REF)), and by compensation means compensating for deviations bychanging the periodic square signals--in response to the actual outputvoltage--which control the AC/DC converter so that it maintains aconstant output voltage.
 7. An emergency power system according to claim2, characterized in that one current path comprises a transistor whoseemitter is connected to the negative electrode of the battery through adiode, which is conductive in a direction toward the emitter, aninductance whose one terminal is connected to the emitter of thetransistor and whose other terminal in connected to the output terminalof the emergency power system through a diode, which is conductive in adirection toward the output, and a transistor which is connected toground.
 8. An emergency power system according to claim 2, characterizedin that the other current path comprises a transistor whose collector isconnected to the positive electrode of the battery through a diode whichis conductive in a direction toward the battery electrode, an inductancewhose one terminal is connected to the collector of the transistor andwhose other terminal is connected to the output terminal of theemergency power system through a diode, which is conductive in adirection toward the inductance, and to a transistor which is connectedto ground.